Significance: Ionizing radiation (IR) is a frequently utilized treatment modality for a variety of cancers, including leukemias, cancers in the pelvic region (e.g. cervical, testicular), and brain or bone malignancies. However, detrimental effects on normal bone marrow (BM) hematopoietic stem and progenitor cells (HSPC) are a major limitation of radiotherapy. We are also faced with the increasing threat of terrorist or military utilization of radioactive/nuclear weapons. Despite intense investigation and need, there is no therapy FDA- approved for broad clinical or emergency hematopoietic radioprotection. Sphingosine 1-phosphate (S1P) is a simple phospholipid found in high nM to low M concentrations in blood and signals via specific G-protein coupled receptors. The best characterized is S1P?, the target of an FDA-approved multiple sclerosis drug, FTY720 (Gilenya). We have recently shown that S1P can signal via S1P?1 on BM lymphocyte progenitors to regulate their proliferation, but roles for S1P? signaling in other HSPC populations, at homeostasis or under stress conditions, are unknown. Preliminary data: With the use of a novel mouse model that reports S1P? activation as nuclear GFP expression, we found that exposure to a single dose of 750 rad gIR results in increased HSPC S1P? expression and signaling. A mouse model of inducible genetic S1P? over-expression (S1P1Tg) yields a BM failure phenotype similar to that induced with ? 900 rad gIR. Administration of clinically relevant doses of FTY720 as little as 2h pre-IR protects from BM failure and subsequent lethality, as does genetic S1P1KO. Based on these data, we hypothesize that increased S1P1 expression and signaling are an HSPC-intrinsic mechanism driving gIR-induced cytotoxicity and apoptosis. DNA damage, mitochondrial dysfunction, and reactive oxygen species are the primary consequences of gIR that trigger cell death, but how S1P? signaling plays a role in these effector pathways is unknown. In the following Specific Aims, we will utilize this novel in vivo model of GPCR signaling and models of inducible S1P1Tg and S1P1KO in ex vivo hematopoiesis and in vivo settings to critically examine the concept that the S1P-S1P1 signaling axis is a major pathway governing these HSPC apoptotic responses to ?-IR: 1) Determine the mechanisms by which gIR induces hematopoietic S1P?1 expression, signaling, and subsequent apoptosis; 2) Determine the mechanism whereby increased S1P? signaling leads to HSPC death and BM failure; 3) Determine if the radioprotective effect of S1P? antagonism leads to increased long-term survival and if there is a coincident increase in leukemia incidence. These studies are likely to have a large overall impact by expanding our understanding of fundamental HSPC apoptotic mechanisms and responses to ?IR, and characterizing a novel, therapeutically tractable signaling pathway as a potential target for prevention of radiation- induced hematotoxicity.